In recent years, with the rapid development of lithium-ion battery, the demand of a variety of portable electronic products and communication tools for lithium-ion battery increases gradually, and the large-scale dynamical lithium-ion power supply also develops rapidly. Cathode material is an important part of a lithium-ion battery, and the research on novel cathode material has become the key to determine the development of lithium-ion battery. At present, the large-scale commercial LiCoO2 has the problems of comparatively large toxicity, high price and security issues. Although LiNiO2 has lower cost and higher capacity, it is hard to be produced and has poor thermal stability and huge problem of security. Spinel LiMn2O4 has low cost and good security, but it has low capacity and poor cycle performance in elevated temperature. In order to satisfy the increased market demand, it is necessary to develop new low-price cathode material with excellent performance. The orthogonal olivine LiFePO4 for new lithium-ion battery cathode material has high capacity and steady voltage of charge and discharge, and especially its low price, good security, good thermal stability and environmentally friendly property make it one of the most potential cathode material.
LiFePO4 exists in the form of lithium iron phosphate mineral in nature. It belongs to orthorhombic system (D162h, Pmnb) and has an olivine orderly structure. There are four LiFePO4 units in each crystal cell, wherein the cell parameters are expressed as follows: a=6.0089 Å, b=10.334 Å and c=4.693 Å. In LiFePO4, the oxygen atoms are approximately hexagonal close packing (hcp), and the phosphorus atom locates in the interval of the tetrahedron, and iron atom and lithium atom locate in the interval of octahedron respectively. The octahedrons of FeO6 connect in common points on the b-c plane of crystal. One octahedron of FeO6 shares common edge with two octahedrons of LiO6, while one tetrahedron of PO4 shares common edge with one octahedron of FeO6 and two octahedrons of LiO6. Li+ has two-dimensional mobility and it can get out and embed during the process of charge and discharge. The strong covalent bonds of P—O form three-dimensional chemical bonds of delocalization, so that LiFePO4 has strong stability of thermodynamics and dynamics. Whereas, the pure phase of LiFePO4 has the weakness of low electronic and ionic conductivity.
In addition, the price and the batch stability are two important factors in order to make lithium iron phosphate commercial available, which however haven't yet attracted sufficient attention currently. The current major methods for synthesizing lithium iron phosphate are the solid-state reaction method at high temperature, the liquid phase co-precipitation method, the hydro-thermal method, the liquid phase redox method, the solid phase micro-wave sintering method, the mechanical ball milling method and etc. The solid-state reaction method in high temperature is a widely used method at present, which using the ferrous iron salt as iron source, then mixing with lithium and phosphorus source, and then sintering in the inert atmosphere to synthesize lithium iron phosphate. However, the cost of the product is greatly increased due to the high price of ferrous iron source, and in order to avoid oxidation of ferrous iron source, the preparation process becomes more complex and it is hard to control the purity of the product. Furthermore, the general time of sintering is more than 6 hours when using this method to prepare lithium iron phosphate.
In order to solve the problem that pure phase LiFePO4 has the weakness of low electronic and ionic conductivity, the researchers did a lot of research. Chinese patent CN100377392C discloses the lithium iron phosphate cathode material LiFe1-xMxPO4-yNz having oxygen vacancy for the secondary lithium battery, wherein M is Li, Na, K, Ag, Cu. It uses N to replace O or uses the monovalent ion to replace Fe in LiFePO4 to improve the electronic and ionic conductivity performance of LiFePO4 to some extent. Chinese patent CN1328808C provides a nitrogen phosphate cathode material LixAaMmBbPOzNn for the secondary lithium battery, which has been improved the electronic and ionic conductivity of the material by doping in the single or double position of Li and/or M in LiFePO4. Both CN100377392C and CN1328808C uses the solid phase sintering method in high temperature for the preparation of the material, which exists the problems of high energy consumption, long sintering time and instable performance of the product. Chinese patent application CN101279725A discloses a micro-wave quick solid phase sintering method of lithium ion battery cathode material lithium iron phosphate. It uses the ferric iron as iron source to solve the problem that ferrous iron is easy to be oxidized and causes impurity of the product. It also reduces the conventional reaction time of 6-30 h to 20-60 min, which improves the production efficiency, saves the energy consumption, reduces the cost of production, and improves the batch stability. The products prepared by this method have good electrode plate processability, conductivity and electrochemistry.